The present invention concerns the use of a composition containing titanium and iron compounds, in particular decomposition residue which occurs as a production residue in the production of titanium dioxide using the sulphate process, as an adsorbing agent for inorganic and organic compounds, for example compounds containing phosphorus, arsenic, antimony, sulphur, selenium, tellurium, cyano and heavy metal, and methods of removing toxic substances and pollutants from a fluid using said adsorbing agent, methods of producing the adsorbing agent and the adsorbing agent which can be obtained by means of those methods.
Arsenic is a constituent part of our environment, that disseminated predominantly from natural sources and is subject to a perpetual biogeochemical cycle. Thus the greatest occurrence of arsenic in the earth occurs bound in sulphide form in the earth's crust. Arsenic has many interesting properties. Thus the toxic element occurs in many different forms, both organic and also inorganic. On a worldwide basis around 200 millions of people are exposed to arsenic-contaminated water. Tumour diseases such as for example skin and lung cancer can be the consequence of increased absorption of arsenic. Arsenic exhibits its carcinogenic action in particular after chronic absorption of small amounts over many years.
Earlier arsenic was used as a fertiliser so that many areas of ground are still contaminated. Arsenic is a problem in particular in Asiatic countries. The main nutrients there include rice which grows in water. If that water is heavily arsenic-bearing then the plant accumulates the toxic substance. Whereas on average only 25 grams of rice per day are consumed in Europe where the arsenic content in drinking water is limited to ten micrograms per liter, in Asia the consumption is 300 grams. Here levels of concentration of 800 micrograms of arsenic per kilo have already been found rice. In algae for example the arsenic is increased in content by the factor of 100000 in comparison with sea water. Thus 180 milligrams of organic arsenic per kilogram of algae (dry weight) have already been found. The plants which are extolled as a miracle health cure from the sea can also contain up to 40 milligrams of inorganic arsenic per kilo of algae dry weight. Unlike the situation for drinking water there is no limit value in Germany for foodstuffs so that those algae may be freely sold.
One possible way of reducing the introduction of arsenic into the food chain involves eliminating the arsenic from drinking water or water which is used for agriculture or livestock breeding or at least reducing the concentration thereof.
Iron oxides and hydroxides have a high affinity for arsenic and are therefore used as adsorbing agents. For example Schlegel et al. describe a granular material comprising iron oxide and hydroxide with a high specific surface area (50 to >200 m2/g) which is highly suitable for the adsorption of heavy metals from fluid flows (A. Schlegel et al., European patent EP 1582505 B1). Adsorbing agents produced specifically for that application are offered for example by Lanxess under the name Bayoxide E33 (with BET surface areas in the range of between 120 and 250 m2/g) and Evers under the name Everzit As (with a BET surface area of >300 m2g).
US patent No 2006/0144793 describes titanium dioxide nanoparticles (<100 Å), whose surface is occupied by hydroxyl groups, and describes that the adsorption of heavy metals takes place on hydroxyl groups and therefore the hydroxyl groups are absolutely necessary for adsorption.
Generally a large effective surface area is an aspect of significant advantage for adsorption processes. It is for example possible to produce an effective TiO2-based adsorbing agent with a large surface area from titanium compounds (for example chlorides or sulphates) by means of hydrolysis in the presence of a porous substrate (US No 2003/068683).
In addition a finely divided titanium dioxide from Dow Chemicals (Adsorbsia GTO with a BET surface area in the range of between 200 and 300 m2/g) was proposed for the adsorption of arsenic.
Graver Technologies offers a granulated adsorbing agent (granule size 250-1190 μm) which for the main part contains titanium dioxide but also titanium hydroxide, but no foreign elements in proportions worth mentioning. That product is used for adsorption of heavy metals. According Graver Technologies both the good adsorption kinetics and also the adsorption capacity are advantages of titanium-based adsorbing agents.
Basically it is assumed on the part of the persons skilled in the art that the adsorption action is correspondingly better, in proportion to the adsorbing agent being more finely divided or the BET surface area being larger or the concentration of hydroxyl groups being higher.
A disadvantage with the existing adsorbing agents is the complicated production procedure and the high costs linked thereto for the adsorbing agent, especially as in practice very large amounts of adsorbing agent are frequently required, for example for purifying ground water.
Therefore the object of the present invention is to find an alternative, inexpensive and highly effective adsorbing agent which can replace the known adsorbing agents and which overcomes the existing disadvantages.